1. Field of the Invention
The present invention relates to an ink jet print head that ejects ink droplets to print a print medium, and in particular, to an ink jet print head having a plurality of types of nozzles arranged on the same substrate and through which ink droplets of different sizes are ejected.
2. Description of the Related Art
With the increased operating speed of ink jet printing apparatuses and improved image quality provided by the ink jet printing apparatuses, attempts have been made to reduce the size of droplets ejected by print heads while increasing ejection frequency.
A reduction in the size of ejected droplets requires a reduction in the opening area of each ejection port in the print head. However, the reduced opening area of the ejection port may increase the flow resistance to a liquid in a portion (ejection port portion) that communicates with the ejection port, preventing desired ejection performance and efficiency from being achieved. Thus, ink jet print heads disclosed in Japanese Patent Laid-Open Nos. 2004-042651 and 2004-042652 serve to reduce the flow resistance of the ejection port portion while maintaining the strength of an ejection port forming portion.
Each of the print heads disclosed in Japanese Patent Laid-Open Nos. 2004-042651 and 2004-042652 has a plurality of nozzles through which ink flows. Each of the nozzles has a bubbling chamber 38 that boils ink to generate bubbles and an ejection port portion 36 including an ejection port 37 that is a tip opening of the nozzle through which ink droplets are ejected, as shown in FIG. 15. The ejection port portion 36 allows the ejection port 37 and the bubbling chamber 38 to communicate with each other and is made up of a first ejection port portion 36a and a second ejection port portion 36b which communicate with the ejection port 37. The first ejection port portion 36a and the second ejection port portion 36b constitute a cylindrical space centered around a central axis passing through the center of an electrothermal conversion element 34 and orthogonally to a major surface 32a of an element substrate 32. When the second ejection port portion 36b is cut in a direction parallel to the major surface 32a, the resulting opening of the second ejection port portion 36b is located outside the opening of the first ejection port portion 36a cut in the same direction and inside a cross-section of the bubbling chamber in the same direction. That is, the second ejection port 36b corresponds to a space formed by enlarging the first ejection port 36a in a plane direction.
In the ink jet print head 30 configured as described above, the thickness of the first ejection port portion 36a ensures the strength of a peripheral portion of the ejection port 37. Furthermore, the enlarged space of the second ejection port 36b enables a reduction in the flow resistance of the whole ejection port portion. Thus, even if the nozzle is provided with an ejection port having a small diameter and through which small droplets are ejected, a possible pressure loss in the ejection port portion 36 can be reduced. Furthermore, bubbles can be grown in an ejection direction. As a result, ink droplets can be efficiently ejected.
Such a reduction in the size of ejected droplets enables a reduction in the size of dots constituting an image and in the sense of granularity conveyed by the image. Thus, the droplet size reduction significantly contributes to improving image quality. However, the droplet size reduction has also been found to be disadvantageous in terms of costs, print speed, thermal efficiency, and the like. That is, when the entire area of the image is formed of small dots in order to reduce the sense of granularity, the number of data in the image increases sharply. This tends to increase the scales of drivers and circuits and thus costs. Furthermore, an increase in nozzle length or chip count for high-speed printing also increases the costs. Moreover, to use small dots to achieve a print speed equivalent to that at which an image is formed using large dots, a nozzle driving frequency needs to be increased compared to that required for printing using the large dots. That is, the number of dots formed per unit time needs to be increased. Thus, the thermal efficiency of a printing operation tends to decrease.
Thus, to solve these problems, a technique has been proposed which provides a plurality of types of nozzles through which ink droplets of different sizes are ejected, on the same head substrate so that one of the plural types of nozzles is selected for use depending on the density of the image. For example, a printing method has been proposed which forms small dots using small ink droplets for a low density portion and an intermediate density portion of the image, while forming large dots using large ink droplets for the intermediate density portion and a high density portion of the image. In this case, if two types of droplet sizes, that is, large and small droplet sizes, are available and the ratio of the large dot to the small dot is about 2 to 4:1, a clear image can be printed by connecting the large and small dots together from the low density portion to the high density portion according to the resolution of the image. Thus, one of the dot sizes is selected for formation depending on the density of the image to be printed. This enables the image to be quickly and efficiently formed, allowing the thermal efficiency of the printing operation to be improved.
However, for the conventional print head, which has the plural types of nozzles of different sizes, each having the ejection port portion composed of the first ejection port portion and the second ejection port portion as described above, ejection characteristics may disadvantageously be unbalanced among the nozzles.
This is because in the conventional print head, the ratio of the opening area of the ejection port to the opening area of the opening of the second ejection port portion is fixed regardless of the size of the ejection port. That is, the nozzle through which smaller ink droplets are ejected suffers a more significant variation in the rate of a pressure loss during ejection in connection with a manufacturing error (misalignment at the boundary portion between the first ejection port portion 36a and the second ejection port portion 36b) in the ejection port portion. This is likely to affect ejection performance such as the amount of ink droplets and landing accuracy. Thus, a possible manufacturing error as described above unbalances the ejection performance between the nozzle with the large ejection port and the nozzle with the small ejection port. This may in turn degrade the quality of images formed using a combination of the large and small dots.
Furthermore, the current ink jet printing apparatus has a suction recovery mechanism that forcibly sucks and discharges thickened ink in the nozzle and bubbles mixed into the ink, through the ejection port to recover the ejection performance of the nozzle. However, a possible manufacturing variation as described above sharply increases the flow resistance to the ink in the small ejection port portion, through which small ink droplets are ejected. Consequently, the suction recovery capability may be degraded, that is, old ink in the nozzle cannot be sufficiently discharged. Namely, for the conventional print head, the nozzle through which smaller ink droplets are ejected is more likely to suffer degradation of the suction recovery capability. This may also unbalance the ejection performance among the various nozzles, degrading the image quality.